Cordierite
, )2 | strunz = 09.CJ.10 | dana = 61.02.01.01 Cordierite group | symmetry = 2/m 2/m 2/m Orthorhombic - Dipyramidal | unit cell = a = 17.079 Å, b = 9.730 Å, c = 9.356 Å; Z = 4 | color = Blue, smoky blue, bluish violet; greenish, yellowish brown, gray; colorless to very pale blue in thin section | habit = Pseudo-hexagonal prismatic twins, as imbedded grains, and massive | system = Orthorhombic - Dipyramidal Space Group: C ccm | twinning = Common on {110}, {130}, simple, lamellar, cyclical | cleavage = Fair on {100}, poor on {001} and {010} | fracture = Subconchoidal | tenacity = Brittle | mohs = 7 - 7.5 | luster = Greasy or vitreous | diaphaniety = Transparent to translucent | refractive = nα = 1.527 - 1.560 nβ = 1.532 - 1.574 nγ = 1.538 - 1.578 Indices increase with Fe content. | opticalprop = Usually optically (-), sometimes (+); 2V = 0-90° | pleochroism = X = pale yellow, green; Y = violet, blue-violet; Z = pale blue | streak = White | gravity = 2.57 - 2.66 | melt = | fusibility = on thin edges | diagnostic = Resembles quartz can be distinguished by pleochroism. Can be distinguished from corundum by its lower hardness | solubility = | other = | references = http://webmineral.com/data/Cordierite.shtml Webmineral data }} Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: ( , )2 ( ) to ( , )2 ( ). A high temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the ( , )6 rings. Name and discovery Cordierite, which was discovered in 1813, is named after the French geologist Louis Cordier (1777–1861). Occurrence Cordierite typically occurs in contact or regional metamorphism of argillaceous rocks. It is especially common in hornfels produced by contact metamorphism of pelitic rocks. Two common metamorphic mineral assemblages include sillimanite-cordierite-spinel and cordierite-spinel-plagioclase-orthopyroxene. Other associated minerals include garnet (cordierite-garnet-sillimanite gneisses) and anthophyllite. Cordierite also occurs in some granites, pegmatites, and norites in gabbroic magmas. Alteration products include mica, chlorite, and talc. Cordierite occurs in the granite contact zone at Geevor Tin Mine in Cornwall. Commercial use Catalytic converters are commonly made from ceramics containing a large proportion of cordierite. The manufacturing process deliberately aligns the cordierite crystals to make use of the very low thermal expansion seen for one axis. This prevents thermal shock cracking from taking place when the catalytic converter is used. Gem variety As the transparent variety iolite, it is often used as a gemstone. The name "iolite" comes from the Greek word for violet. Another old name is dichroite, a Greek word meaning "two-colored rock", a reference to cordierite's strong pleochroism. It has also been called "water-sapphire" and "Vikings' Compass" because of its usefulness in determining the direction of the sun on overcast days, the Vikings having used it for this purpose . This works by determining the direction of polarization of the sky overhead. Light scattered by air molecules is polarized, and the direction of the polarization is at right angles to a line to the sun, even when the sun's disk itself is obscured by dense fog or lies just below the horizon. Gem quality iolite varies in color from sapphire blue to blue violet to yellowish gray to light blue as the light angle changes. Iolite is sometimes used as an inexpensive substitute for sapphire. It is much softer than sapphires and is abundantly found in Sri Lanka, India, Burma, Australia's Northern Territory, Namibia, Brazil, Tanzania, Madagascar, Connecticut, and the Yellowknife area of the Northwest Territories of Canada. Geologist Dan Hausel, University of Wyoming, found iolite deposits in Wyoming. These are some of the largest deposits in the world. One iolite that he recovered weighed in at more than 24,000 carats--the largest ever found in the world.[http://www.topix.com/city/casper-wy/2011/09/wyoming-is-most-gemstone-rich-state-in-us Topix Local News: Casper, WY, Wyoming is Most Gemstone-Rich State in US, Sept. 13, 2011] Image:Cordierite.jpg|''Left'': rough specimen showing dichroism; right: cut stone Image:Iolite.JPG|Facet cut Iolite gemstone Image:Cordierite polychroïsme.jpg|Pleochroism of Cordierite See also *List of minerals *List of minerals named after people References }} External links *Mineral galleries *http://www.gemstone.org/gem-by-gem/english/iolite.html Category:Magnesium minerals Category:Iron minerals Category:Aluminium minerals Category:Cyclosilicates Category:Orthorhombic minerals cs:Cordierit de:Cordierit eu:Kordierita fa:کوردیریت fr:Cordiérite it:Cordierite he:קורדיאריט kk:Кордиерит nl:Cordieriet ja:菫青石 pl:Kordieryt ru:Кордиерит fi:Kordieriitti sv:Kordierit tr:İyolit uk:Кордієрит vi:Cordierit